Interesting. I imagine that we may well be talking (dirty) water here (rather than 'insulation'), which may possibly afford some of the explanation, perhaps via electrochemical mechanisms. I wonder what it would have shown at 230V (or even 500V) DC.
That would be interesting - and, depending on what results you get, maybe also ~250V DC.
True. As I said, that implies the ability to measure down to 45 nA (at 9V, even lower if voltage is lower) - which is pretty impressive.
The only analogue meters I have surviving (like my trusty 1960s AVO Multiminor Mk 4, still going strong) couldn't achieve anything like that. On that AVO, the highest marking on the ohms scale, virtually indistinguishable from the bottom end of the scale (needle 'resting point'), is 20KΩ, which on its 'x100' range would equate to just 2MΩ. I think that most analogue multimeters were based on 50μA (sometimes 100μA) movements - so, even with 9V, to be able to measure ~200MΩ would require one to be able to detect a movement of about one-thousandth of full scale deflection.
That's much more iffy. When trying to measure extremely high resistances, at least one can look to see if there is any perceptible needle movement from its 'resting position' when one connects. For very low resistance, one is having to visually judge how close a nearly-FSD reading is to the point one adjusted the zero point to be.
JohnW2";p="2439570 said:
Is that not exactly what one does when, with an analogue meter, one adjusts the reading obtained with the leads shorted to read exactly FSD, and then moves the leads to the resistance one wishes to measure?
Sure, but I don't think that alters the generality of what I've been saying. If one wanted to use relatively small voltages, that constant current would have to be extremely small. I'm not sure how easy is is to design accurate constant current sources in the nA range.
Given that we are talking about messuring resistances of hundreds of MΩ, one would need a voltmeter of incredibly high input resistance for that to give reasonable answers.
Very similar. my method means the needle only moves between dead short and the resistor under test without crossing back across the whole scale between the two readings
I was talking about measuring very low resistance value.
Ah, yes, sorry - this discussion has been jumping around a bit! In that case, my comment turns on its head - the requirement would then be for an ammeter with an extremely low internal resistance (rather than a volmeter with an extremely high input resistance).
Very true but, as I said as an example, in the case of my analogue AVO Multiminor, it would be unreasonable to produce an estimate of anyting higher than ">2MΩ", since even 2MΩ produces only a just-about-perceptable movement of the needle..
Don't all ammeters need an extremely low resistance (ideally zero) and all voltmeters an extremely high one (ideally infinite)?
Of course they do (depending on one's interpretation of 'extremely'), if one wants accurate measurements and minimal interference with what one is measuring.
Interesting though that the closer to the ideal you can get the better your instrument but if you could actually reach the ideal it would be no use at all...
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